Variable resistance and method of making same



Aug. 27. 1940. E. R. STOEKLE 2,213,078

VARIABLE RESISTANCE AND METHOD OF MAKING SAME Filed May 27, 1937 3 Sheets-Sheet l l Y l INVENTOR 4'. BY @www R vmwu \6o2`^- u. ATTORNEY.

VARIABLE RESISTANCE AND METHOD OF MAKING SAME Filed May 27, 1937 3 Sheets-Sheet 2 Manish... v

INVENTOR. wm Q. stoma Aug. 27, 1940. E. R. STOEKLE 2,213,078

VARIABLE RESISTANCE AND METHOD OF MAKING SAME Filed May 27, 1957 3 Sheets-Sheet 3 if 5/5701? C06 rnv@ A TTORNEY.

Patented Aug. 27, 1940 UNITED STATES PATENT OFFICE VARIABLE RESISTANCE AND DIETHOD OF MAKING SAlVIE tion of Delaware Application May 27, 1937, Serial No. 144,997

3 Claims.

This invention relates to a variable or adjustable resistance ofthe type especially adapted for employment as a volume or tone control instrumentally in radio receivers, and to a method of making Vsucha resistance.

One of the objects of the invention is to provide a variable resistance of this character which is much smaller and far more compact and more closely organized than previously known devices of this character and yet is as equally effective as the larger devices heretofore made, known, or used. In particular, the variable resistance embodying the present invention has maximum dimensions approximately one-half lthose of devices heretofore known and yet has the capacity and effectiveness of similar devices twice its size.

In carrying out the invention the resistance element is so constituted as to be capable of standing up under the maximum amount of heat generated by virtue of the passage of current therethrough and yet is combined in such a novel manner with its metal casing that although the resistance element is electrically insulated from the casing it is in good or efcient heat interchanging relation therewith and the casing itself is constituted to aid in the dissipation of the heat transmitted thereto from the resistance element. In the performance of its function of dissipating the heat, the casing may be and preferably is aided by the mounting bushing and the structure on which it is supported.

The invention further proposes a novel type of adjustable or rotatable contact constituted so as to be radially resilient and otherwise adapted for eeotive cooperation with the carbonaceous coating or conductive surface of the resistance element. The adjustable contact is combined with its operating shaft and with a stop arrangement in a simple and effective manner. The invention also provides a novel way of insulating and mounting the terminals and electrically connecting them with the ends of the resistance element.

In its various embodiments the invention provides one type of structure in which the adjustable or rotatable contact element is directly connected to its operating shaft and consequently it, as Well as kthe resistance, is electrically connected to the casing and its bushing, and also another type of structure wherein the adjustable contact andthe resistance are electrically insulated from the metal housing and its bushing, provisionbeing made in the `latter instance for the simple and advantageous connection of the adjustable contact in the circuit in which the device is incorporated or with which it is associated.

Other objects and advantages reside in certain novel features of the construction, arrangement and combination of parts which will be hereinafter more fully described and particularly pointed out in the appended claims, reference being had to the accompanying drawings forming a part of this specification, and in which:

Figure 1 is a plan View on an enlarged scale of a variable resistance embodying the present invention of the type in which the adjustable contact is directly Xed to its voperating shaft, parts being broken away for the sake of illustration;

Figure 2 is a sectional view taken on line 2-2 of Figure 1, parts being shown in elevation for the sake of illustration;

Figure 3 is a detail view in top plan showing the stop plate combined with the operating shaft;

Figure 4 is a similar detail view of the adjustable or rotatable contact;

Figure 5 is a View in bottom plan showing the insulating cover plate, its terminals and the contact arms associated with .the terminals;

Figure 6 is a view similar to Figure 1 but `showing an embodiment of the invention in which the adjustable contact is electrically insulated from its operating shaft and therefore from its bushing and the housing or casing;

Figure 7 is a sectional View taken on line 'l-'l of Figure 6;

Figure 8 is a View in top plan showing theadjustable contact employed in the embodiment ofA the invention shown in Figures 6 and 7;

Figure 9 is a view in bottom plan of the cover plate and terminal arrangement utilized in the form of the invention shown in Figures 6 and 7;

Figure 10 is a detail View in top plan of the stop plate employed with this form of the invention;

Figure 11 is a fragmentary view on an even greater enlarged scale taken partly in transverse section and partly in elevation and illustrating the manner in which the resistance element is combined with the metal casing;

Figure l2 is a diagrammatical view partly in side elevation and partly in longitudinal vertical section showing a machine which may be emcontrol "in a radio-receiver. kxthe resistance-element 6 andthe inner curved wall of the body portion 2 of the metal housing is a layerof electrically insulating but heat con- I Way of combining the resistance element with the metal housing or casing.

The embodiments of the invention shown in Figures 1 to 11 are illustrated on a scale four times their actual size to clarify the illustration and make the drawings easier to read. The two embodiments of the invention shownin Figures 1 to 11 have certain features in common and these will first be described. In Vboth of these embodiments the variable resistance comprises a metal casing or housing designated generally at l and having a cylindrical wall or body portion 2, and an end wall 3 integral with one end of thecylindrical wall 2. The other end of the cylindrical wall 2 is open. Theend wall 3 is provided with a central opening to receive an end portion of an externally threaded bushing 4. The end of the bushing adjacent the wall 3 of the housing is provided with an outwardly directed and integral annularflange 5. This ange 5 is secured to the end wall ofy the housing in any Suitable Way but preferably by projection welding, that is to say, small, inwardly extending projections are provided on the end wall @adjacent its central opening `and these'projections are fused or melted down under pressure and the iniiuence of -a'fheavy welding currentf'By welding the parts together in'this manner, the bushing is effectively securedto the housing and an eicient heat interchanging connection is established therebetween. The same result could be b- `tained by machining or' stamping the housing and its bushing out ofone piece, and to some extent theadvantages may be had by staking the'bushing" tol the housing in the conventional fashion, but preferably: the parts are separably .formedaJnd-llledefl'JOgether the manner described.

To the inner curvedwall of the body portion 2 ofthe metal housing a resistance element designated generally at' 6 is applied. The resistance element 6 is made'up cfa-'thin strip of dense heat resisting paper (about .005 to .007 thick) y which'paper; duefto its density, has better than the average thermal conductivity of ordinary paper' although able ltostand vup underthe heat conditions towhich 'it-is subjected. A suitable paper vfor-thisl-'purpose is'theA so-called armature slot paper that-i's-treated rag base paper commonly used for insulating the conductors of electrical machinesin theirslots. A resistance coating of carbonaceous material is applied to one surface of this paper, preferably in the so- A called tapered resistance manner, well known in the art,'to` meet the resistance variations requiredto get the desired volume control or tone Interposed between f strip.

there isa gap 'between ,its adjacent ends. The

The length of the `resistancestrip or element 6 isvsuch that when-assembled with the housing,

adhesive layer is however .continuous and unportion2' of the casing and is somewhat wider thanV theresistance element 6 to prevent the possibility of current leakage Ato the metal case.

vDue to the enlarged scale on lwhich the draw- '4. ings are madegjthe ,adhesive insulating layer 1 appears tofbeofs'orne substantial thickness. Ac-

to effect this operation.

tually the layer 1 is quite thin, so much so that the effect so far as heat conduction is concerned is practically the same as though the resistance element Were in direct contact with the inner wall of the housing. Of course, however, there is suicient of the insulating adhesive layer 1 to electrically insulate the resistance element 6 from the metal case.

The invention may be carried out by coating the inside walls of the housing with a suitable 10 adhesive, such as an insulating japan, and when this is in a semi-dry or tacky condition the resistance element is curled with its resistance coating on the concave inner surface of the cylindrical Wall of the body portion 2 of the metal 15 housing thereby causing it to adhere to the insulating coating of japan. Further drying and baking of the japan coating by suitably controlled means will then cause the resistor to adhere firmly to the curved inner wall of the hous- 20 ing with a minimum'thickness of Vinsulating material between the wall of thehous'ing and the resistance surface. I c vL Another way of combining the resistance Velement with the casing consists in coating the inner surface of the housing with a solutionof a thermo-plastic resin or equivalent adhesive which is allowed to dry completely before inserting the resistance element. After the resin -has dried, a strip of the resistance material is curved to conform to the inner cylindrical wall rof .the housing and its uncoatedsurface is pressed, by an expandible plug,.iirmly against the lmor layer of the resin or adhesive while-the housing is heated to a temperature sumcient to soften the thermo-plastic coating `and cause adherence and intimate contact between the resistance element and the adhesive coated Wall ofthe housing. Here again, while the resistance element is securely held in position itis electrically insulated ,40

pose a thin strip of thermo-plastic resin; or a` thin piece of paper impregnated withr said `resin between the uncoated surface of the resistance element and the inner Wall of the housing, the interposed strip being slightlywider and larger than the resistance element to insure eiective.

electric insulation.V When the resistance element, strip and casing or housingare assembled in the manner described the resistance element l is pressed firmly against the strip and the strip against the inner wall of the housing by means.55

of e an expanded plug while the housing isheatedA suiiiciently to cause softening of the thermoplastic strip and consequent intimate adhesion of the resistance element to the metal housing.

The present invention proposes to heat ther thermo-plastic strip or lm to its softening point" by heating the walls of the metal vhousing just long enough to effect the desirable heating of the thermo-plastic strip or film without heating the resistance element itself to any appreciablel err-65 Excessive heating of the resistance ele-"` tent. ment is likely to cause a changein its resistance. To avoid this and at the same time obtain rapid production the present Vinvention proposes to heat the strip by exposing the assembly of thepgo casing, strip and resistance element tov a high frequency electromagnetic field for a short time. In other words, the well known high frequency electrical induction method .of heating is adapted In Figure 12 an apparatus for carrying out this mode of heating the assembly is diagrammatically illustrated.

The apparatus shown in Figure 12 comprises a base IIJ on which a carrier plate II is slidably supported, the carrier plate being constrained by suitable guides or ways to slide back and forth across the base I0 and having its movement in either direction limited by stop pins I5 and I5. On the carrier plate I I are a pair of similar holders designated at I2 and I3. These holders are of identical construction and each is provided with a vertically extending central opening in which a hollow pin I4 is fitted, each pin I4 projecting down into an opening provided therefor in the carrier plate II. An assembly of the casing I, bushing 4, thermo-plastic strip I and resistance element 6 is mounted in the holder engaged with either the stop pin I5 or I 6, the bushing 4 of the unit extending down into the hollow pin I4 thereof as shown in Figure 12. In the example illustrated in Figure 12 the assembly just referred to has been rst associated with the holder I2. The carrier plate I I is then slid over until the holder I3 engages the stop pin i6, and the holder l2 with the assembly mounted thereon is disposed beneath and in vertical alignment with the pilot 20 of the ram 2l of a press. The ram of the press is then lowered by manipulation of a conventional foot pedal (not shown) until the rubber plug 22 guided by the pilot 20 extending into the bushing 4 presses the resistance element 6 and also the insulating strip 'I against the inner wall of the body portion 2 of the casing I with a maximum pressure determined by spring and stop (not shown) in the press. Simultaneously with the lowering of the ram 2! a circuit is closed by spring supported contacts (not shown) coacting with the ram of the press. These contacts when closed permit electric current from a high frequency generator (not shown) to pass through the flexible leads 23 to an induction heating coil 24 for a time which is accurately determined by the heating of a timing device (not shown) on the magnetic switch (not shown) which controls the high frequency generator. While this operation is being completed on the unit in one holder (the holder I2 in Figure 12) the operator mounts another unit in the holder I3 which at this time is not under the ram of the press. Since the time required to complete the heating of a unit is only a second or so this cycle of operation makes for maximum production. It will be noted that the radial pressure exerted by the rubber plug 22 results from the Ypressure exerted on it by the conical surfaces 25 and 26 formed on the lower end of the ram 2I and on the flange 21 of the pilot 20, the plug being interposed between these conical surfaces and the pilot 2li being supported for limited sliding movement in the bore of the ram.

As previously indicated the high heat dissipation resulting by thus mounting the resistance in intimate thermal relation with although electrically insulated from the metal case makes it possible to reduce the size of the metal casing of the resistance to approximately one-half of that formerly used for the same electrical load. This brings the diameter of the unit capable of dissipating one Watt or more down to five-eighths of an inch.

A hollow operating shaft 3l) is supported for rotation in the bushing 4 in the embodiments of in veach instance being limited in its inward movement by means of a split annular spring washer 3l snapped into an annular groove 32 provided in the shaft and engageable with the outer end of the bushing. The inner end of the shaft is employed in each instance to rotate the movable contact, but as the structure from this point on in these two embodiments differs, their remaining elements will be separately described.

In the structure shown in Figures 1 to 5 the movable contact is designated generally at 35 and has a disk-like body portion 36 having its inner periphery notched as at 3I to adapt it to interlock with the inner end of the operating shaft 3i? when the inner end portion is staked into engagement with the movable contact and with its cooperating stop plate 38 which is disposed inwardly of the movable contact. The stop plate 33 has similar notches 37 for the same purpose.

Integral with the disk-like body of the movable contact is a spring Contact member 39 in the form of an approximately U-shaped loop with the ends of the loop merging into the peripheral portion of the body of the movable contact and the bite of the loop shaped to present a contact pad 4I).

This construction, while simple and readily and inexpensively fabricated supplies the movable contact 35 with resiliency required and well adapts it in other respects for use with a unit of this type. The pad 40 of the movable contact'in the assembly has wiping contact with the carbonaceous coating or conductive surface of the resistance. Between the underside of the body portion 35 of the movable contact 35 and the adjacent flanged end of the bushing 4 suitable washers designated at 4I and 4I are interposed. By staking the inner end of the shaft in the movable contact 35 and its stop plate 38 these parts are permanently secured to the shaft and are constrained to rotate therewith. The stop plate 33 has a radial projecting lug 42 which is engageable with a cooperating stop lug 43 which may be conveniently struck up from the end wall 3 of the metal case. These lugs limit the rotation of the movable contact in either direction and prevent its contact pad 4U from riding off either end of the resistance element.

The end wall 3 of the metal case may have an insulating washer 44 applied thereto.

The open end of the metal case is closed by a disk-like cover 45 of insulating material, the cover being held in place by tabs 46 which may be integral with the body portion of the metal case and be bent over the circular edge of the cover as shown in the drawings. In this form of the invention the cover 45 carries two of the terminals designated at 4l and 48. The third terminal 43 may be integrally formed with the case in this embodiment of the invention since the movable contact and consequently the resistance element are not insulated from the operating shaft but are electrically connected to this shaft and consequently to the bushing 4 and case i.

The terminals 41 and 48 are of identical construction although oppositely arranged, Each includes a slotted portion 50 projecting externally of the cover and formed at the outer end of an inclined intermediate portion 5I which extends through slots 52 in the cover and has a fiat inner portion 53 integral therewith, disposed on the inner side of the cover and riveted thereto as at 54. Integral with the flat portions 53 are curved contact arms 55, the ends of which are shaped and adapted to bear flush against the end portion of the carbonaceous coating or conductive surface of the resistance element 6. They are, however, located above and out of the path of travel of the Contact pad i0 and adjacent portions of the movable contact.

In the form of the invention shown in Figures 6 to l0 inclusive the movable or rotatable contact associated with the inner end of the operating shaft 30 is designated generally at 5l). It has an approximately disk-shaped body portion 6I having a lateral extension 32 with which a curved contact member or arm 53 is integrally formed. The portion of the arm 53 engageable with the resistance is curved or bowed as at 63 to provide a contact pad or button. rIhe body portion Si of the adjustable contact rests flatly on an insulating Washer tbl and is interconnected thereto so as to be constrained to rotation therewith by means of lugs 55 integral with the body portion and extended at right angles therefrom and into peripheral notches S6 formed in the insulating washer et. The insulating washer ll has notches 556 on its inner periphery to receive the displaced portions 97 of the inner end of the shaft which is staked .into engagement with its insulating washer and with a similarly hatched stop plate t8. The stop plate 58 is vdisposed on the oppositeside of the washer from the adjustable contact di. A'washer S59 is interposed between the stop plate t8 and the adjacent flanged end 5 ofthe bushing fl. The stcp plate 58 has astop lug l@ which is cocperable with a stop lugli struck upfrom the end wall 3 of the case or housing. These stop lugs 'lil and 'il function to limit the angular rotation of the shaft and adjustable contact in either direction. -It will be noted that while the shaft is staked into mechanical engagement or interlocking rela- 'cover` 'i2 bymeans of a rivet l5.

tionship with the Washer @l and the stop plate 63 it is spaced from the adjustable contact E0 and is electrically insulated from this contact Eil' by the insulating Washer 6G.

A cover plate 'l2 of insulating material is provided for the open end of the casing l in this form of the invention and as in the other structural type is held in place by tabs or lugs "i3 integral with the body portion 3 cf the casing and bent over the periphery of the cover plate in the assembly. v

Terminals fil and l are carried by and cornbined with the cover plate '52. These terminals All and t8' are identical with the terminals il and i8 previously described and are combined with their cover' plate in exactly the same manner in which the terminals il and S are combined With-their cover plate d5. These terminals dl and fifi have contact arms 55 corresponding to the contact arms 55 and coop-erable with the resistance element 6 in just the same way as the contact arms 55.

In the Figures 6 and 7 the casing is shown as having a third terminal i9 integral therewith because `the casing shown is identical in the two embodiments of the invention shown in Figures 1 to 1l. However, in the embodiment shown in Figures 6 to 10 this third terminal :i9 has no useful function and may be dispensed with. An active or effective third terminal designated at l5 is provided and is supported on the insulating'cover l2, the terminal l5 being riveted to the An angular portion 'W of the third terminal l5 extends through a slot i8 provided in the cover plate 12 and its inner extremity is bifurcated and provided with contact buttons 7S which bear or wipe against the body portion 6I of the movable contact BU. By supporting the third' terminal and the adjustable contact in an insulating manner and electrically connecting them in the manner shown and described, the adjustable contact may be conveniently incorporated in or associated with an electric circuit while being insulated from the operating shaft, its supporting bushing, and the metal case or housing.

The construction shown in Figures 6 to 10 provides a fully insulated potential divider or potentiometer type of resistance such as are most commonly used in radio circuits.

Figure 13 illustrates a specically different way of combining or positioning the resistance element in the casing of a variable resistance of the type here under consideration.

In Figure 13 the casing is designated at 80 and has a cylindrical wall 8| and an integral end Wall 82. A bushing 83 is suitably combined with the end Wall 82 and resistance and provides the support for the operating shaft 84. A spring contact arm 85 is fixed to the inner end of the shaft 84 and its outer end is connected to a contact member 36 which wipes against a resistance element 8l. This resistance element 31 consists of a thin strip of paper of flexible materialof the character previously described and provided on its upper or exposed face with a conductive coating such as a coating of carbonaceous material. An adhesive layer of the character previously described and designated at 88 is interposed between the resistance yelement and the end wall 82 of the casing. In this case we have a flat annular resistance element and it is applied to the end Wall instead of the curved or cylindrical Wall of the casing or housing. As in the other embodiments of the invention this resistance element although electrically insulated from the case by the adhesive layer is so intimately related thereto as to be in adjacent heat interchanging relation with the case.

With this type of structure a iiat circular platen of a press will serve to stick the resistance .and adhesive layer firmly to the metal endor bottom of the case while the latter is heated bysuitable means to secure the desired assembly and interrelation of the parts.

From the foregoing it Will be understood that the present invention involves a novel method of combining a resistor with a metal case and while the method may be carried out in the various Ways above described, in each instance it contemplates placing a resistor in the case with the uncoated surface of the resistance strip adjacent the inner metal wall of the case and with the carbonaceous coated or conductive surface of the strip facing inwardly. Interposed between the uncoated surface ofthe resistance strip and the adjacent wall of the metal case is a layer of electrically insulating but heat conductive adhesive, as described, which may take various forms. Withthe parts of the resistance thusassembled the resistor or resistance strip is pressed outwardly toward the adjacent wall of the housing and the housing is heated to soften the thermoplastic resin or other adhesive with the result that the resistor is caused to adhere firmly to and have intimate thermal-conductive relation with the wall of the metal casing although electrically insulated therefrom. In this way rapid dissipation of heat from the resistor is insured and the electrical load capacity of the resistor is increased.

While I have shown and described several constructions in which the variable resistance con- ;75

stituting the present invention may be embodied and illustrated also several methods of making the resistance, it is to be understood that these particular' descriptions are in many respects illustrative only and that various forms and rnodiications within the spirit of the invention and the scope of the subjoined claims are contemplated as will be apparent to those skilled in the art.

The invention claimed is:

1. A variable resistance of the character described comprising a metal casing having a curved wall, a resistor comprising a thin strip of heat resistant material having a conductive coating on one side thereof, said resistor being curved to conform to the curved inner wall of said casing with the coating exposed to the inside of the housing, said resistor being or such length as to provide a gap at the ends thereof when so placed, a thin electrically insulating adhesive layer interposed between said strip and the metal wall and being co-eXtensive of said gap and causing the strip to adhere to the wall whereby the strip although electrically insulated from the wall is in effective heat interchanging relationship therewith.

2. A variable resistance or the character described comprising a metal casing, a resistor comprising a thin base of good thermal-conductivity and a conductive resistance film on one side of said base, said base being mounted with its uncoated surface adjacent an inner surface of said metal casing, a gap between the ends of said base, an electrically insulating adhesive of good thermal-conductivity interposed between and cementing the uncoated side of the base to the surface or" the metal casing to which said base is adjacent and covering the surface of said casing at said gap, and an externally threaded metal bushing fused to a wall of the casing whereby the bushing as well as the metal casing takes part in the dissipation of the heat generated in the resistance by the passage of electric current therethrough and transmitted to the casing by virtue of the good thermal conductivity of the electrically insulating adhesive.

3. A variable resistance of the character described comprising a metal casing, a resistance element directly applied to but electrically insulated from the inner wall of the casing, an operating shaft supported on the casing, an insulating washer secured to said shaft for rotation therewith, a movable contact having a circular portion interconnected with the washer for ro- 'f tation therewith but electrically insulated from the shaft and having a curved contact member bea-ring against said resistance element, an insulating cover for said casing, a pair of terminals mounted on said cover and having contact arms engageable with the ends of the resistance element, and a third terminal also supported on said insulating cover and having a bifurcated inner portion provided with contact pads bearing against the circular portion of the movable contact.

ERWIN R. STOEKLE. 

